The present invention relates to circuitry for controlling the power level provided to a load from a periodic waveform electric power source.
In many applications, such as stage and television lighting, it is desirable to provide a power control circuit which maintains a predetermined power output to the load under all conditions. For example, by maintaining a minimum energization of a lamp load, the lamps can be made to respond quickly when the operator desires to increase the light output, since the lamp filaments are not permitted to cool down completely. In certain applications, such as television lighting, it is desirable to maintain a minimum light output just above the threshold of the television camera's response.
In different applications and, for a given application under different conditions, the desired relationship between the position of the operator actuated control and the power output to the load varies. For example, in stage lighting applications, it is known that the human eye generally perceives light intensity to vary approximately in proportion to the square of the RMS output voltage to the lamp. However, the eye's sensitivity to changes in light levels varies inversely with the ambient light level. For television lighting, on the other hand, it is found that the camera reacts quite differently than the eye to changes in light intensity, thus necessitating yet another functional relationship between operator control position and the output voltage to the lamp.
In the prior art, various methods have been attempted to provide the design flexibility necessary for adapting the control circuitry to such variable conditions of operation. In one method, this functional relationship consists of a piecewise linear approximation to the desired response curve. In order to closely approximate the desired response curve, it is necessary to produce an actual response consisting of a relatively large number of linear segments. This technique, this, results in relatively complex circuitry and a commensurately high cost of production.